This invention relates to aqueous dispersions of polyamide resins and more particularly, to aqueous dispersions of polyamide resins having improved stability against gelation and phase separation.
Polyamide resins are well known as a class of resins, as are numerous methods for their preparation. Polyamide resins are typically manufactured by reacting a di- or polyfunctional amine with a di- or polyfunctional acid. Most commonly-employed diacids and diamines yield polyamide resins which are essentially linear. The properties of polyamide resins will vary considerably, depending upon the particular synthetic reactants employed. Polyamide resins which are prepared from relatively short chain diacids and diamines having, for example, 5-10 carbon atoms will tend to be relatively crystalline and have excellent fiber forming properties. These types of polyamide resins are typically referred to as nylons.
Polyamide resins are also prepared from relatively long chain polyfunctional acids and diamines. A particularly important class of polyamide resins of this type are referred to as polymerized fatty acid polyamide resins. The polymerized fatty acid polyamide resins are especially useful in products such as hot melt adhesives, water resistant coatings, and printing inks because of their physical properties, including high strength, excellent flexibility, water and solvent resistance, and the ability to form smooth, non-tacky coatings.
The polyfunctional acids used in the preparation of polymerized fatty acid polyamide resins are derived from higher unsaturated fatty acids by polymerization. In the polymerization process, the fatty acids having double bond functionalities combine to produce mixtures of higher polymeric acids. The polymerized fatty acid polyamide resins are, in turn, typically prepared by reacting one or more suitable diamines--most commonly relatively short chain diamines--with the polymerized fatty acid. Often, another diacid is also reacted to increase the softening point or other properties. The polymerized fatty acid polyamide resins which are obtained tend to be more amorphous than the nylon type of polyamides resins and are generally more flexible. The differences in the physical properties of the polymerized fatty acid polyamide resins as compared to the nylon type of polyamide resins are considered to be related to the long chain length of the polymerized fatty acid component.
The polymerized fatty acid polyamide resins are widely used in a variety of industrial applications. Polymerized fatty acid polyamides are especially useful as hot melt adhesives and for forming water and solvent resistant coatings on substrates such as paper. An important related use of polymerized fatty acid polyamide resins is as binders in printing inks and the like where film toughness, flexibility, and adhesion are important properties.
One of the problems encountered with the polyamide resins--particularly the polymeric fatty acid polyamides--relates to the methods used to apply the resins to substrates. One method which has been suggested involves heating the polyamide resins above their melting point and then applying the molten resins to the substrate. This technique, however, has certain inherent problems. For example, polyamide resins typically have high melting points, often higher than the thermal stability of the substrates onto which they are to be applied. Accordingly, the hot melt method can only be used in certain limited applications which require relatively expensive application equipment. Thus, the use of molten polyamide resins is not practical in applications such as, for example, printing. Molten polyamide resins are also impractical where the resin is to be applied as a latent hot melt layer to be activated at a later time. For example, it may be desired to apply a polyamide resin to a textile interliner, incorporate the interliner into a garment, and then activate the adhesive to hold the assembled parts of the garment in position.
It has been recognized that certain of the problems associated with the polyamide resins might be solved if the polyamides could be applied at ambient temperatures as a solution or a dispersion. For many applications, however, solutions of polyamide resins are unsatisfactory. Polyamide resins as a class have excellent resistance to solvents; even with respect to those solvents in which the polyamide resins are soluble, the solubility typically is relatively low. Furthermore, the solvents which have been used to make polyamide resin solutions often adversely react with the substrates to which the polyamide resin solutions are applied. A further problem associated with solvent solutions is that most solvents used are relatively expensive, often difficult or impossible to remove from the applied coatings, and present fire, toxicity, and environmental pollution problems.
To overcome or at least reduce the problems associated with such solvent solutions, it has been suggested to prepare emulsions or dispersions of the polyamide resins in water. Early emulsions were prepared by initially dissolving the polyamide resin in an organic solvent and then using selected emulsification agents to form an emulsion of the solvent solution and water. The resulting solvent/water polyamide resin emulsions still had the problems associated with the presence of solvents and were relatively unstable. In addition, films formed from these emulsions tended to have an undesirable tackiness. Those skilled in the art will appreciate that instability is manifested in aqueous resin dispersions by phenomena such as phase separation or undesired interparticle interactions resulting in agglomeration, better known to those skilled in the art as gelation.
In British patent 1,491,136 there was disclosed a method for forming aqueous dispersions of various plastic powders, including polyamide resin powders. In the disclosed method, the polymer resin was first mechanically reduced to a powder form and then blended with water and a thickening agent. The method was less than satisfactory. The mechanical reduction of the resins to the required particle size was both expensive and difficult to control and often caused thermal degradation of the polymers. Furthermore, the resulting thickening dispersions had limited utility in many applications because of the relatively high viscosity and the presence of the thickening agent.
It is also known to render a polyamide resin more readily dispersible in water by chemically modifying the resin so as to include solubilizing groups. This includes, for example, incorporating alkoxymethyl groups, as disclosed in U.S Pat. No. 2,430,860 (Carirns) and U.S. Pat. No. 2,714,075 (Watson, et al.). However, the incorporation of the additional groups into the polyamide resin increases the cost of the polymer and also typically reduces the desirable properties of the polyamide resins, especially in relation to water and solvent resistance.
Another known method for increasing the water dispersibility of polyamide resins involves formation of a resin having a considerable excess of either free carboxyl or free amine groups. At least a portion of the free acid or free amine groups are then neutralized to form salt groups on the polyamide resin, which salt groups act as internal surfactants to facilitate the dispersion of the modified polyamide in water. In U.S. Pat. No. 2,811,459 (Witcoff, et al.) there is disclosed a method for preparing polymerized fatty acid polyamide dispersions wherein the polyamide is formed from a substantial excess of a diamine. The resulting polyamide resins are then dispersed in an aqueous solution of an acid so that the acid forms salt groups which act as an internal surfactant to allow formation of an aqueous dispersion In U.S. Pat. No. 2,768,090 (Witcoff, et al.) a similar process is disclosed wherein the excess amine groups of a polyamide resin are reacted with an acid to form intrinsic ammonium salt groups and, hence, a cationic dispersion which is converted to an anionic dispersion by charge inversion. A similar salt forming process utilizing free amino groups was disclosed in U.S. Pat. No. 2,824,848 (Witcoff). In U.S. Pat. No. 2,926,117 (Witcoff) there is disclosed a method wherein the polyamide resin formed with a deliberate excess of acid groups is then dispersed in an aqueous medium containing an alkaline substance to cause formation of salt groups which act as internal surfactants.
The discussed methods for preparing aqueous dispersions of polymerized fatty acid polyamides having salt groups are relatively effective in initially forming aqueous dispersions. However, the dispersions have limited stability and are not satisfactory for use in many applications, as their synthesis requires the presence of substantial amounts of free acid or free amino groups which adversely effect the performance properties of the dispersed polyamide resin. Optimal properties are typically achieved by conducting the amidations so as to cause as complete as a reaction as possible. This requires that approximately stoichiometric amounts of the starting diacid and diamine be employed and that the reaction be conducted so as to produce a final product having a low amine number and low acid number. The presence of substantial excesses of either reactant or an incomplete reaction--as required for the prior art salt forming polyamide material--inherently reduce the chain length and the resulting strength and flexibility of the polyamide resin.
Furthermore, incorporation of polymers having substantial excess amounts of unreacted polymerized fatty acids typically results in unstable materials. The fatty acids can be liberated from the polymer and cause exceptional tackiness and undesirable degradation of the desired properties of the polyamide resin. These polyamide resins continue to react during application, which causes increases in molecular weight and coating viscosity, as well as changes in the melting point. A still further problem encountered with the method wherein the salt forms of the polyamide resins are used is that the salts tend to decompose during application and the resulting material when applied becomes undesirably tacky. This is particularly undesirable in many applications, such as in printing inks and protective coatings.
The stability of aqueous dispersions of polyamide resin may in certain applications be improved by the use of casein and other thickening agents and in many cases may cause gelation. However, thickening agents only slow down phase separation. These materials are retained in the resin and may have undesirable application properties.
Because of the problems associated with the polymerized fatty acid polyamide resins having large amounts of salt groups formed as part of the polymer, the aqueous dispersions of these particular types of resins have had no substantial commercial success.